1. Technical Field
The present disclosure relates to start-up circuits of portable electronic devices, and particularly to a start-up circuit used to start-up central processing units (CPU) of portable electronic devices.
2. Description of Related Art
Portable electronic devices, such as mobile phones, personal digital assistants (PDA) and laptop computers, are widely used. A portable electronic device usually has a CPU installed therein. When the portable electronic device is used, the CPU needs to be activated first for controlling components of the portable electronic device.
FIG. 4 shows a circuit diagram of a conventional CPU start-up circuit 90 of a portable electronic device (not shown). The CPU start-up circuit 90 can activate and turn off a CPU 80 of the portable electronic device, and can also control the CPU 80 to reset. The CPU start-up circuit 90 includes a power management unit (PMU) 91, a main power supply 92, a subsidiary power supply 93 and an awaking switch 94, wherein both the main power supply 92 and the subsidiary power supply 93 can be conventional power supplies of the portable electronic devices. The PMU 91 includes at least two ports 911, 912. The ports 911, 912 are both electrically connected to the CPU 80. The main power supply 92 and the subsidiary power supply 93 are both electrically connected to the CPU 80 and PMU 91. The main power supply 92 can provide working electric power to the CPU 80, and the subsidiary power supply 93 can provide working electric power to the clock of the CPU 80. The awaking switch 94 is electrically connected to the CPU 80. The PMU 91 can generate a reset signal (RS) and a status signal (SS) respectively transmitted to the CPU 80 through the ports 911, 912 to control the CPU 80. Generally, the electric potentials of both the RS and the SS have high levels (e.g., higher than about 2.0V) and low levels (e.g., lower than about 0.8V). The CPU 80 receiving an SS at the high level can be started-up to work. If the working CPU 80 receives an SS at the low level, the CPU 80 is automatically turned off. The working CPU 80 remains to work when receiving an RS at the high level, and resets when receiving an RS at the low level. The turned off CPU 80 receiving an SS at the high level can be switched on by a waking signal (WS) sent from the awaking switch 94.
In use, the main power supply 92 supplies working electric power to the portable electronic device, and both the main power supply 92 and the subsidiary power supply 93 can supply working electric power to the PMU 91. The PMU 91 detects the working status of the main power supply 92 and generates corresponding RS and SS. The subsidiary power supply 93 can provide pull-up voltages to the ports 911, 912, thereby regulating the electric potentials of the RS and the SS into predetermined ranges. When the main power supply 92 works normally, the PMU 91 generates an RS and an SS both at high levels and transmits the RS and the SS to the CPU 80. Thus, the CPU 80 can be switched on by operating the awaking switch 94, and the main power supply 92 can provide working electric power to the CPU 80. When the main power supply 92 is removed or has not sufficient working electric power, the PMU 91 generates an RS and an SS both at low levels and transmits the RS and the SS to the CPU 80, such that the CPU 80 resets and is then switched off. However, the PMU 91 may generate a high-impedance status on the port transmitting the SS (i.e., the port 912) when the main power supply 92 is removed. The high-impedance status may cooperate with the pull-up voltage provided by the subsidiary power supply 93 to form a high electric potential on the port 912. Thus, the CPU 80 may mistakenly identify that the port 912 outputs an SS at the high level. If the switch 94 is mistakenly operated/selected, a WS may be generated and transmitted to the CPU 80. Thus, the CPU 80 will be mistakenly activated, and the portable electronic device may be damaged.
Therefore, there is room for improvement within the art.